A fail-safe r-c phase shift oscillating type of level detector

ABSTRACT

A fail-safe level detector comprising a regenerative feedback oscillator and a voltage breakdown device interconnecting the output and common electrodes of the oscillator so that sufficient feedback for sustaining oscillations only occurs when a D.C. input causes the breakdown device to conduct and to assume its low dynamic impedance condition.

United States Patent Darrow May 28, 1974 1 A FAIL-SAFE R-C PHASE SHIFT[56] References Cited OSCILLATING TYPE OF LEVEL UNITED STATES PATENTS2,905,835 9/1959 Wray 331/117 X [75] Inventor: John O. G Darrow,Murrysville, P 3,054,971 9/1962 Khu 4. 331/108 B [73] Assignee:Westinghouse Air Brake Company, Primary Examiner Herman Karl Saalbachswlssvale Assistant Examiner-Siegfried H. Frimm [22] Filed: Sept. 7,1972 Attorney, Agent, or FirmH. A. Williamson; J. B. 211 App]. No.:287,103 Sotak Related US. Application Data 57 ABSTRACT [60] Division P121 1970 A fail-safe level detector comprising a regenerative Commuauo"of 706,914 201.1968 feedback oscillator and a voltage breakdown deviceabandoned interconnecting the output and common electrodes of theoscillator so that sufficient feedback for sustaining [52] 1 21 3 1oscillations only occurs when a DC. input causes the l breakdown deviceto conduct and to assume its low [51] Int. Cl. 1103b 5/20 dynamicimpedance condition [58] Field of Search 331/108 B, 109, 135-137, 1

6 Claims, 2 Drawing Figures A' I 6'10 19 21? 17 T Z 1440 cz 18 $66 14 AFAIL-SAFE R-C PHASE SHIFT OSCILLATING T PE QELEVEL DE R.

This application is a division of my copending application for LettersPatent of the U.S., Ser. No. 1,970, filed Jan. 12, 1970, for Fail-SafeCircuit Arrangements. now US. Pat. No. 3,737,806. Application Ser. No.1,970 is a streamlined continuation of application Ser. No. 706,914,filed Feb. 20, 1968, for Fail-Safe Circuit Arrangements, now abandoned.

My invention relates to fail-safe level detectors and more particularlyto fail-safe circuit arrangements employing a feedback oscillator andthe voltage and dynamic impedance characteristics of a breakdown devicefor providing an AC. output when and only when the amplitude of a DCinput exceeds a predetermined value.

In various automatic control systems safety is of catastrophicimportance. For example, in vehicle speed detection arrangements formass and/or rapid transit operations, it is mandatory to determine theactual speed of a moving vehicle and thereafter to compare the actualspeed with the prescribed speed command request for a given area orsection in order to prevent injury to individuals and damage toequipment. That is, in such systems, it is a preemptory safetyrequirement that under no circumstances should the actual speed of themoving vehicle exceed the preselected speed command request for anygiven area. In one particular arrangement, the actual speed of a movingvehicle is derived by suitable speed sensing apparatus, such as an axledriven frequency generator which delivers A.C. signals having afrequency directly proportional to the vehicular velocity. These A.C.signals obtained from the frequency generator are, in turn, applied to asuitable voltage limiter which prevents an excess voltage swing ineither direction and provides that the signal amplitudes will besubstantially constant. These limited signals are, in turn, applied to asuitable fail-safe lowpass filter which is selectively chosen to have anupper frequency limit corresponding to speed command request for theparticular area. Accordingly, the filter will only pass signals havingfrequencies below the upper frequency level. It will be appreciated thatas the vehicle moves from one area to the next, the upper frequencylimit may be automatically controlled by varying the filter componentsand their values or by selecting one of a plurality of low-pass filtercircuits in accordance with the prescribed command request for eachgiven area. The A.C. output signals taken from the lowpass filter may,in turn, be converted by a fail-safe rectifier to provide a DC. outputvoltage which is proportional thereto. Accordingly, if it is desired toinsure that a vehicle is proceeding at a speed below some preselectedvalue, it is merely necessary to measure the amplitude of the DC. outputsignals supplied by the rectifier. However, as in all vital portions ofsuch speed command control systems, this measuring function must beperformed by fail-safe apparatus which will not provide an output signalwhen the vehicle is moving in excess of the preselected command request.That is, it is of the utmost importance to exercise extreme care indesigning and constructing this portion of the apparatus in order tomaintain the security and integrity of the overall system. Accordingly,it is readily evident that the detecting apparatus must operate in afail-safe manner so that any conceivable and foreseeable failure willresult in a condition at least as restrictive and preferably morerestrictive then that preceeding the failure. For example, in suchapparatus a circuit malfunction or component failure should not bepermitted to erroneously simulate or indicate a condition for holding ormaintaining the vehicles speed, and normally, it is pre ferred that thefailure should either provide a warning such as flashing a light,sounding a buzzer, or intitate a braking action for stopping thevehicle. Thus, in order to insure a higher degree of safety toindividuals as well as apparatus, it is necessary and essential thatunder no circumstances will a failure cause or be capable of simulatinga true or valid speed indication.

Accordingly, it is an object of my invention to provide a new andimproved fail-safe circuit arrangement.

A further object of my invention is to provide a unique amplitude leveldetector circuit which operates in a fail-safe manner.

Another object of my invention is to provide an improved semiconductivecircuit arrangement which will provide an output, when and only when theamplitude of an input exceeds a predetermined value.

Yet another object of my invention is to provide a new fail-safe leveldetector for measuring the amplitude of an input and only producing anoutput when the amplitude of the input exceeds a predetermined level.

Still another object of my invention is to provide a fail-safe circuitarrangement formeasuring a DC. input and for providing an AC. outputwhen and only when the amplitude of the DC. input exceeds thepreselected value.

Still yet another object of my invention is to provide a fail-safeamplitude level detector employing the dynamic impedance characteristicsof a breakdown device for controlling the conductive condition of aregenerative circuit.

Still yet a further object of my invention is to provide atransistorized level detector which operates in a failsafe manner toproduce an output when and only when an input exceeds a predeterminedlevel.

A still further object of my invention is to provide a feedback type ofoscillator which is only capable of sustaining oscillations when aninput causes a voltage device to breakdown and assume its low dynamicimpedance condition.

Yet still another object of my invention is to provide a fail-safecircuit arrangement which is simple in construction, economical in cost,efficient and reliable in operation.

Briefly, the fail-safe level detector of the present invention employs afeedback type of oscillator circuit and a voltage breakdown device. Theoscillator includes a transistor amplifier and a frequency determiningcircuit. The frequency determining circuit is interconnected by thevoltage breakdown device to the transistor amplifier for controlling theamount of regenerative feedback and in turn the oscillating condition ofthe oscillator. Normally, the voltage breakdown device exhibits a highdynamic impedance and only assumes a low dynamic impedance conditionwhen a sufficient DC. voltage causes the device to breakdown andconduct. Accordingly, the oscillator will only sustain A.C. oscillationswhen a DC. voltage exceeds a predetermined amplitude for causing thebreakdown device to conduct and assume its low impedance condition sothat sufficient regenerative feedback is provided from the output to theinput of the oscillator.

in accordance with one embodiment of the invention, the fail-safe leveldetector employs a Colpitts type of oscillator.

In accordance with another embodiment of the present invention, thefail-safe level detector utilizes an RC phase-shift oscillator.

The foregoing objects and other attendant features and advantages willbe more readily appreciated as the subject invention becomes betterunderstood by reference to the following detailed description whenconsidered in conjunction with the accompanying drawings wherein:

FIG. 1 is a schematic circuit diagram of a fail-safe level detector inaccordance with the present invention wherein the oscillator comprises aColpitts type of oscillator.

FIG. 2 is a schematic circuit diagram of a fail-safe level detector inaccordance with the present invention wherein the oscillator comprisesan RC phase-shift oscillator.

As is well known, a feedback type of oscillator may be defined as atuned feedback amplifier in which the amplitude and phase angle of thefeedback signal are such as to cause sustained oscillation. That is, thefeedback signal must be in phase with the input and must be ofsufficient magnitude to overcome the normal circuit losses in order tosustain the oscillations. ln oscillating circuits of this type, itisconventional practice to connect the frequency determining circuitbetween the input and output electrodes of the amplifier and also toconnect the frequency determining circuit to the common electrode of theamplifier. As will be described in greater detail hereinafter, bycontrolling the impedance characteristic of the common electrodeconnection, the amount of regenerative feedback and in turn the givenlosses therein may be employed to control the conductive condition ofthe amplifier. That is, a high value of impedance appearing in thecommon connection will proportionally increase the negative ordegenerative feedback which therefore materially reduces the gain of theamplifier thereby effectively dampening the oscillations of theoscillator. The present invention makes use of this operating principlein a unique manner wherein the condition of an oscillator is effectivelycontrolled and an AC. output is only available when the magnitude ofaD.C. input exceeds a predetermined value.

Referring now to the drawings and in particular to FIG. 1, there isshown a fail-safe amplitude level detector characterized by thenumeral 1. As previously mentioned, the amplitude voltage level detector1 operates in a fail-safe manner to measure the amplitude of a DC inputvoltage which is representative of the actual speed of the movingvehicle. That is. the DC. voltage supplied by the speed sensingapparatus is applied to the input terminals 3 and 4. As shown, a currentlimiting resistor R] has one of its ends connected to the input terminal3 and the other end connected to the cathode of the Zener diode Z]. Theanode of the Zener diode Z1 is connected to the other input terminal 4which is appropriately grounded thereby forming a common terminal. Theresistor R] and Zener diode Z1 form a voltage regulator which stabilizesand provides a substantially constant operating and biasing supplyvoltage for improved operation. In addition to its voltage regulatingfeature, the voltage breakdowndevice or Zener diode also exhibits theunique characteristics of having a high dynamic impedance condition whennot conducting and of having a low dynamic impedance condition whenrendered conductive and operating properly. As will be described ingreater detail hereinafter, this unique characteristic is employed inthe present invention for insuring that the level detector operates in afail-safe manner.

A voltage dividing network consisting of series connected resistors R2,R3 and R4 is connected in parallel across the Zener diode Z1. Theresistive values of the voltage dividing network are selected to providethe proper biasing potentials for the various electrodes of thetransistor T1 which forms the amplifier or active element of a Colpittsoscillator. The transistor T1 in .cludes a common emitter electrode 7,an output collector electrode 8 and an input base electrode 9. Thefrequency determining circuit is in the form of a parallel tuned orresonant tank circuit 10 which is tuned to a predetermined frequency andcomprises series connected capacitors C2 and C3 connected in parallelwith inductor L1. As shown, one end of the tank circuit 10 is directlyconnected to the collector electrode 8 while the other end of the tankcircuit 10 is directly connected to the junction point 5 of the voltagedividing network. The base electrode 9 is directly connected to thejunction point 6 of the voltage dividing network. A signal passingcapacitor C1 is connected between the junction points 5 and 6 of thevoltage dividing network thereby connecting the other end of theresonant tank circuit 10 with the base electrode 9. The common point 11of the two tuning capacitors C2 and C3 is directly connected to theupper portion of the voltage divider network and, in turn, to thecathode of the Zener diode Z]. The emitter electrode 7 is connectedthrough resistor R5 to ground. A by-pass capacitor C4 is connected inparallel with resistor R5. The collector electrode 8 is connectedthrough the coupling capacitor C5 to the output terminal 13. The outputterminals 13 and 14 may be connected to a suitable utilization device,such as a vital type of under-speed relay after being appropriatelyamplified and rectified.

Turning now to the operation, it will be assumed that the moving vehicleis proceeding at or below the preselected speed command request so thatthe DC. signals generated by the speed sensing apparatus and applied tothe input terminals 3 and 4 are of a preselected or predetermined level.As previously mentioned, the speed sensing apparatus is only capable ofgenerating a sufficient level of DC. voltage when the vehicle isproceeding at a speed lower than a preselected speed command request forthe particular area. The voltage breakdown characteristic of the Zenerdiode Z1 is appropriately selected to require a potential levelsubstantially equal to the voltage level of the sensing apparatuswhen'the vehicle is moving below the preselected speed command request.Accordingly, under this condition the DC. input voltage is of thesufficient magnitude to cause the Zener diode Z] to breakdown therebycausing the diode to conduct and exhibit a low dynamic impedance. Sincethe voltage across the Zener diode remains substantially constant over awide range of voltage and current changes, the various biasing voltagessupplied by the voltage dividing network insure stable operation of thetransistor oscillator. With the Zener diode conducting, a low impedancepath is established from the common point 11 of the tuned resonantcircuit to the emitter 7 of the transistor amplifier TI. This pathextends from the common point 11, through the Zener diode Z1, throughby-pass capacitor C4 to the emitter electrode 7 of the transistor T1.Accordingly, sufficient regenerative feedback is now provided for thetransistor oscillator so that oscillations occur and an A.C. outputsignal is present on the output terminals 13 and 14. As is well known,the A.C. output power which is available at the collector electrode 8 isa function of the amplifier gain minus the feedback power. As previouslymentioned, this A.C. voltage on terminals 13 and 14 after suitableamplification and rectification may, in turn, be employed to energizethe underspeed relay thereby indicating that the speed of the vehicle isnot in excess of the preselected command request. That is, the presenceof A.C. output at the terminals 13 and 14 may be construed as a true orvalid indication that the vehicle is proceeding at or below thepreselected speed command request.

Let us now assume that the actual speed of the vehicle increases to apoint beyond the predetermined speed command request so that the circuitoperation may be analyzed under this condition. Under this condition,the axle generator now produces a signal of increased frequency. Thesehigher frequency signals are greatly attenuated due to the inherentrejection characteristics of the low-pass filter, so that the DC. outputproduced by the rectifier network is proportionally reduced at thistime. Accordingly, the level of DC. input voltage applied to terminals 3and 4 is substantially below the Zener threshold or breakdown voltage ofthe diode Z1. Now with an insufficient magnitude of DC. voltage appliedacross the Zener diode Z1, the diode will not conduct and will exhibit ahigh dynamic impedance. In view of the high impedance condition of theZener diode Z], the feedback path from the junction point 11 of the tankcircuit 10 to the emitter 7 of transistor T1 will appear as an opencircuit and therefore the oscillator will not oscillate. In practice,the impedance of resistor R1 is chosen to be relatively high so thatsufficient regenerative feedback cannot occur through the speed sensingapparatus. Accordingly, under this condition no A.C. output is availableat the terminals 13 and I4, and therefore the under-speed relay becomesdeenergized thereby signifying that the vehicle is now proceeding at thespeed above the preselected speed command request for the given area.The deenergization of the under-speed relay may, in turn, cause theenergization ofa suitable alarm, such as lighting a lamp or sounding abuzzer, or may initiate an automatic braking action to slow down or stopthe vehicle entirely.

As previously mentioned, the amplitude level detector must operate in afail-safe manner so that no conceivable component or circuit failurewill be capable of producing an A.C. output on the output terminals 13and 14 at this time. It will be noted that if the Zener diode becomesshort-circuited, the necessary biasing or supply voltages are notavailable and therefore the oscillator is incapable of oscillating. lfthe Zener diode becomes open-circuited, it is quite apparent that therequired low impedance path between the junction point 11 and emitter 7is not present so that oscillations cannot occur and no A.C. signal isagain available at the output terminals 13 and 14. if the Zener diodebecomes leaky and conducts at some voltage lower than the normalbreakdown voltage, the dynamic impedance exhibited by the diode is stillgenerally sufficient to cause an appreciable amount of degeneration sothat oscillations will not occur. An open-circuit failure of the currentlimiting resistor R1 is obviously a safe condition. Normally,fail-safeness is based on the premise that resistors or resistiveelements cannot become short-circuited due to the particular type ofresistors, namely, carbon-composition, employed in circuits which mustoperate in a fail-safe manner. It will be noted that the various othercomponents and elements constituting the oscillator circuit will eitherfail in a safe manner or destroy the circuit integrity to the pointwhere oscillations will not occur. Accordingly, it will be observed thatthe presently described level detector operates in a fail-safe manner sothat an A.C. output is available at the output terminals 13 and 14 whenand only when a predetermined value of DC. input is applied to the inputterminals 3 and 4.

Referring now to FIG. 2, there is shown an alterna tive embodiment of afail-safe level detector employing the inventive concept advancedherein. The circuit of HG. 2 is in general similar to that of FIG. 1,and similar components have been designated by primed numerals. In thisembodiment the fail-safe level detector which is characterized bynumeral 12 is slightly different from the detector of FIG. 1 in that anRC phase-shift oscillator is employed as the oscillating circuit. As inFIG. 1 the output from the speed sensing apparatus is connected to theterminals 3 and 4', respectively. The input terminal 3' is coupled toone end of the carbon composition type of current limiting resistor R1similar to that of FIG. 1. The other end of resistor R1 is connected tothe cathode of Zener diode Z1 whose anode is connected to the inputterminal 4'. The phaseshift oscillator includes an amplifying devicesuch as transistor T2 having an input base electrode 17, a commonemitter electrode 18, and an output collector electrode 19. The baseelectrode 17 is connected to the negative 12 volt terminal ofa suitablesource of biasing potential by resistor R6. The emitter electrodel8 isconnected to the common terminal or ground through a potentiometer oradjustable resistor R7 which may be selectively varied to control thenecessary gain of the amplifier. The collector 19 is connected throughload resistor R8 to the cathode of the Zener diode Z1. ln order toprovide the necessary degree phase shift which is required for thecommon-emitter configuration, an RC phase shift or frequency ofoscillation determining network is employed for making the feedbacksignal positive when returned from output to input. The resistancecapacitance phase-shifting network consists of three sections, eachcontributing approximately a 60 phase shift at the frequency ofoscillation. The three sections consist of C R C R and C and itsassociated effective resistive impedance which in effect is resistanceR8. As shown, the junction of capacitor C and R is directly connected tothe base electrode 17 while the other side of capacitor C is directlyconnected to the cathode of Zener diode Z1. The output terminals 13 and14' are connected to the collector electrode 19 by coupling capacitor C5and to ground, respectively.

The function and operation of the amplitude level detector of FIG. 2 aresubstantially identical to that of FIG. 1 in that the phase shiftoscillator can only produce oscillations when the Zener diode Z1conducts and exhibits low dynamic impedance condition. That is, a lowimpedance feedback path is established from the common junction ofcapacitors C C and C to the emitter 18 of transistor T2 under thiscondition. However, when the DC. voltage applied to terminals 3' and 4'is insufficient to breakdown the Zener diode, the oscillator will notoscillate and deliver an A.C. output on terminals 13 and 14 due to thehigh impedance presented by Zener diode Z1. The signal voltage developedacross the high impedance of the Zener diode Z1 causes degenerativefeedback to occur so that the gain of the amplifier is reduced and henceoscillations will not occur. As previously mentioned, the uniquecharacteristics of the Zener diode Z1 also prevent oscillations fromoccuring when the diode becomes shortcircuited or open-circuited due toeither the absence of the necessary biasing and operating voltages orthe interruptionof the A.C. feedback connection, respectively. Aspreviously mentioned, a leaky Zener diode fails in a safe manner in thata relatively high dynamic impedance accompanies a conducting Zener diodewhich avalanches at a reduced voltage level.

Accordingly, an A.C. output is only available at terminals 13' and 14when the applied DC. voltage on terminals 3' and 4' is of a sufficientmagnitude to cause the Zener diode Z1 to become conductive andassume itslow dynamic impedance condition. Then and only then will the feedbacksignal be in phase with the input and the amplifier gain be sufficientto overcome the feedback losses so that the oscillator will sustainoscillations and supply an A.C. output signal of the selected frequencyon the terminals 13' and 14. As previously mentioned, the output takenfrom terminals 13' and 14' may be employed to energize an under-speedrelay in an automatic speed control system or alternately may beutilized to supply switches, gates or other elements in various logiccircuits.

Accordingly, it will be noted that each of the above described fail-safeamplitude level detectors functions and operates in a fail-safe mannerto produce an A.C. output when and only when the magnitude of a DC.input exceeds a preselected value.

Further, it may be mentioned the circuit parameters of each of thedescribed have been selected to preferably employ a Zener diode having athreshold" or breakdown voltage in the range of 6 to 8 volts in that nopresently known diode in this range can conduct at a lower than ratedZener voltage and yet exhibit a low dynamic impedance. However, it isreadily understood that Zener diodes having other voltage Zener ratingsmay equally well be employed when it is possible to positively insurethat when the diode breaks down at a lower than its rated Zener voltage,it will not exhibit a low impedance condition.

Also it will be appreciated that while the present invention has beendescribed in terms of Zener diodes and transistor amplifiers, it isreadily understood that other voltage stabilizing or breakdown devicessuch as neon glow or gas regulator tubes and that other amplifyingdevices such as gas or vacuum tubes may be employed with equal success.

Although common-emitter configurations and NPN transistors have beenillustrated, it is understood that common-collector or common-baseconfigurations as well as transistors of opposite conductivity, that is,PNP transistors may be used in practicing the present invention bymerely reversing the polarity of the directcurrent input and of theZener diode, as is well known.

In addition, it will be appreciated that while Colpitts and RC phaseshift types of oscillator circuits have been illustrated in practicingthe present invention, it is readily understood that various other typesof oscillator circuits such as Hartley, Clapp and numerous other typesof feedback oscillators may be equally used as the oscillation producingcircuit. Similarly, it is readily understood that LC or RL sections maybe used in place of the RC sections in the phase-shift oscillator, andthat the capacitors and resistors of the phase-shift network may beinterchanged with equal success.

It will also be appreciated that while this invention finds particularutility in speed control systems, it is readily evident that theinvention is not merely limited thereto but may be employed in variousother systems and apparatus which require the security and safetyinherent in the invention. But regardless of the manner in which theinvention is used, it is understood that various alterations may be madeby persons skilled in the art without departing from the spirit andscope of this invention. It will also be apparent that othermodifications and changes can be made in the presently describedinvention, and therefore it is understood that all changes andequivalents and modifications within the spirit and scope of thisinvention are herein meant to be included in the appended claims.

Having thus described my invention, whatlclaim is:

l. A fail-safe level detector comprising, a transistor oscillationcircuit having an input, and output, and-a frequency determining networkselectively connected between said input and said output, biasing meansconnected to said input and lead means connected to said output and avoltage breakdown device electrically connected across said input forregulating said input and also electrically connecting said frequencydeter mining network from said output to said input for therebycompleting a feedback circuit whereby regenerative feedback is presentwhen said voltage breakdown device conducts and assumes a low impedancecondition so that said oscillating circuit produces A.C. oscillationswhen and only when said breakdown device is conductive and is operatingproperly so that it exhibits a low impedance condition which permitssufficient feedback to maintain said A.C. oscillations.

2. A fail-safe level detector as defined in claim 1, wherein saidoscillating circuit includes a solid-state amplifier and an RCphase-shifting network.

3. A fail-safe level detector as defined in claim 1, wherein saidoscillating circuit comprises a phase-shift type of transistor feedbackamplifier.

4. A fail-safe level detector as defined in claim 1, wherein saidvoltage breadkown device is a Zener diode.

5. A fail-safe level detector as defined in claim 4, wherein said Zenerdiode is connected as a shunt regulator.

6. A fail-safe circuit arrangement comprising, an input, an output and acommon terminal, a currentlimiting resistor and a Zener diodeelectrically connected between said input and said common terminals, atransistor having an emitter, a collector and a base electrode, a firstresistor coupling said collector electrode to the junction of saidcurrent-limiting resistor and said Zener diode and a capacitor couplingsaid col- 10 curs and A.C. oscillations are produced on said outputterminal when and only when a DC. voltage of sufficient amplitude ispresent on said input terminal to cause said Zener diode to conduct andassume a low dynamic impedance condition.

1. A fail-safe level detector comprising, a transistor oscillationcircuit having an input, and output, and a frequency determining networkselectively connected between said input and said output, biasing meansconnected to said input and lead means connected to said output and avoltage breakdown device electrically connected across said input forregulating said input and also electrically connecting said frequencydetermining network from said output to said input for therebycompleting a feedback circuit whereby regenerative feedback is presentwhen said voltage breakdown device conducts and assumes a low impedancecondition sO that said oscillating circuit produces A.C. oscillationswhen and only when said breakdown device is conductive and is operatingproperly so that it exhibits a low impedance condition which permitssufficient feedback to maintain said A.C. oscillations.
 2. A fail-safelevel detector as defined in claim 1, wherein said oscillating circuitincludes a solid-state amplifier and an RC phase-shifting network.
 3. Afail-safe level detector as defined in claim 1, wherein said oscillatingcircuit comprises a phase-shift type of transistor feedback amplifier.4. A fail-safe level detector as defined in claim 1, wherein saidvoltage breadkown device is a Zener diode.
 5. A fail-safe level detectoras defined in claim 4, wherein said Zener diode is connected as a shuntregulator.
 6. A fail-safe circuit arrangement comprising, an input, anoutput and a common terminal, a current-limiting resistor and a Zenerdiode electrically connected between said input and said commonterminals, a transistor having an emitter, a collector and a baseelectrode, a first resistor coupling said collector electrode to thejunction of said current-limiting resistor and said Zener diode and acapacitor coupling said collector electrode to said output terminal, asecond resistor coupling said emitter electrode to said common terminal,a third resistor coupling said base electrode to the negative terminalof a supply source, and a resistance-capacity phase-shifting circuitcoupled from said collector electrode through said Zener diode to saidemitter electrode whereby regenerative feedback occurs and A.C.oscillations are produced on said output terminal when and only when aD.C. voltage of sufficient amplitude is present on said input terminalto cause said Zener diode to conduct and assume a low dynamic impedancecondition.